Studies of Phenothiazine. X. Further Observations on Oxidation of Phenothiazine. ∗

Several years ago an interesting observation was made on the oxidation of phenothiazine in the dry state. At that time no practical importance was attached to the observation, and it was not reported. As will be shown later, recent developments on the use of phenothiazine as a fungicide have pointed to the importance of this observation, which has now been repeatedly confirmed, and deemed worthy of a report.

A porcelain mortar and pestle had been used for intimately mixing a small quantity of phenothiazine with the basic rat diet routinely used in this laboratory. Subsequently the same mortar was wiped out, but not washed, and used for mixing bentonite with rat diet. It was observed that after a few minutes of mixing the whole contents of the mortar had assumed a pink tint. Realizing that traces of phenothiazine might have been left in the mortar, and believing that an oxidation to thionol could account for the color developed, the following experiments were performed.

The mortar and pestle were thoroughly washed, and a sample of bentonite alone was thoroughly ground. No coloration developed. After washing the mortar and pestle, phenothiazine alone was ground and again no color developed. The experiment was then repeated with a mixture of phenothiazine and bentonite. After a few moments of grinding the mixture showed a definite pink color which rapidly developed into a deep brick-red as the grinding proceeded. Repetition of the experiment by other members of the staff yielded the same result. Substitution of talc for bentonite gave the same results, but the color development was somewhat slower, indicating that talc was less effective than bentonite in facilitating the oxidation of phenothiazine.

To determine whether or not the grinding was an essential part of the oxidation mechanism, a sample of bentonite and phenothiazine was placed in a test tube and the contents were mixed by repeated but gentle inversions of the tube.